1. Field of the Invention
The present invention relates to solar cells. In particular, the present invention relates to methods and apparatuses for mounting a diode on a solar cell.
2. Description of the Related Art
Photovoltaic cells, commonly called solar cells, are well-known devices which convert solar energy into electrical energy. Solar cells have long been used to generate electrical power in both terrestrial and space applications. Solar cells offer several advantages over more conventional power sources. For example, solar cells offer a clean method for generating electricity. Furthermore, solar cells do not have to be replenished with fossil fuels. Instead, solar cells are powered by the virtually limitless energy of the sun. However, the use of solar cells has been limited because solar cells are a relatively expensive method of generating electricity. Nonetheless, the solar cell is an attractive device for generating energy in space, where low-cost conventional power sources are unavailable.
Solar cells are typically assembled into arrays of solar cells connected together in series, or in parallel, or in a series-parallel combination. The number of cells in an array, as well as the array topology, is at least in part determined by the desired output voltage and current.
As is well-known in the art, when all cells in an array are illuminated, each cell will be forward biased. However, if one or more of the cells is shadowed (i.e., not illuminated), such as by a satellite antenna, the shadowed cell or cells may become reversed biased because of the voltage generated by the unshadowed cells. Reverse biasing of a cell can cause permanent degradation in cell performance or even complete cell failure. To guard against such damage, it is customary to provide protective bypass diodes. One bypass diode may be connected across several cells, or, for enhanced reliability, each cell may have its own bypass diode. Multijunction solar cells are particularly susceptible to damage when subjected to a reverse bias condition. Thus, multijunction cells in particular benefit from having one bypass diode per cell. Typically, a bypass diode is connected in an anti-parallel configuration, with the anode and the cathode of the bypass diode respectively connected to the cathode and the anode of the solar cell, so that the bypass diode will be reversed biased when the cells are illuminated. When a cell is shadowed, current through the shadowed cell becomes limited and the shadowed cell becomes reverse biased. The bypass diode connected across the shadowed cell in turn becomes forward biased. Current will flow through the bypass diode rather than through the shadowed cell, thereby allowing current to continue flowing through the array. In addition, the bypass diode limits the reverse bias voltage across the shadowed cell, thereby protecting the shadowed cell.
Several different prior art methods have been used to provide solar cells with bypass diode protection. Each prior art method has its drawbacks. For example, in an attempt to provide increased bypass protection, one method involves locating a bypass diode between adjacent cells, with the anode of the bypass diode connected to one cell and the cathode of the diode connected to an adjoining cell. However, this technique requires that the cells be assembled into an array before the bypass diode protection can be added. This assembly method is difficult and inefficient. Furthermore, this technique requires the bypass diodes to be added by the array assemble rather than by the cell manufacturer. In addition, this technique requires the cells to be spaced far enough apart so as to accommodate the bypass diode. This spacing results in the array having a smaller active area, and thus the array is less efficient on an area basis.
Another prior art technique providing a bypass diode for each cell requires that a recess be formed on the back of the cell in which a bypass diode is placed. Each cell is provided with a first polarity contact on a front surface of the cell and a second polarity contact is provided on a back surface of the cell. An "S" shaped interconnect must then be welded from a back surface contact of a first cell to a front surface contact of an adjoining cell. Thus, this technique disadvantageously requires the cells to be spaced far enough apart to accommodate the interconnect which must pass between the adjoining cells. In addition, rear-mounted diodes typically protrude a significant amount from the solar cell backside. Thus, when adhering the solar cell/bypass diode assembly to a panel, a very thick, heavy layer of adhesive must be applied to the solar cell/bypass diode assembly backside so that the assembly will lay flat on the panel. The added weight of the adhesive is very disadvantageous for space-based applications. Furthermore, the present prior art technique requires the connection of the interconnect to the adjoining cell to be performed by the array assembler as opposed to the cell manufacturer.
Still another prior art technique for providing a bypass diode involves mounting a bypass diode on the front of a solar cell, with one diode contact connected to a contact on the back of the solar cell using a discrete C-clamp type interconnect and one diode contact connected to a contact on the front of the solar cell. This technique requires flipping the solar cell from front to back to weld or solder the interconnections to the front and back solar cell contacts. The flipping process often damages the cell, greatly reducing manufacturing yields. Furthermore, this technique disadvantageously requires adjoining cells to be spaced far enough apart to accommodate the C-clamp type interconnects which must pass between the adjoining cells.